Glass fibres can be produced by melting a glass composition in a melter and drawing them out through the tip plate of a bushing assembly. A bushing assembly is generally in the form of a rectangular box with two pairs of opposed side and end walls forming the perimeter thereof with an opening at the top in fluid communication with the melter. The bottom floor comprises a tip plate which is a plate comprising a multitude of orifices or tips through which the glass melt can flow to form fibres, which are attenuated to reach their desired diameter. To ensure an optimal control of the viscosity, the temperature of the tip plate must be controlled accurately. The temperature of the tip plate depends on the glass melt composition but is generally well above 1200° C. Because of the extreme working conditions, the various components of a bushing assembly are made of platinum or platinum alloys, typically rhodium-platinum alloys.
The bushing is heated electrically by passing current through the body of the bushing from a first connector clamped to a first terminal ear, electrically coupled to a first end wall of the bushing body, to a second connector clamped to a second terminal ear, thermally coupled to a second end wall of the bushing body, opposite the first end wall. A wide variety of terminal ear geometries have been proposed in the art. It can be said that in practically all cases they comprise at least a thin plate extending transverse from a bushing end wall. To prevent the connectors, usually made of copper, from overheating and from deforming, they are usually water cooled. The terminal ears are therefore exposed to severe thermal gradients, between the free end thereof, where water cooled connectors are clamped and the end coupled to the bushing end wall which is at temperatures of well above 1200° C. Such extreme temperature gradients have two drawbacks; first the cooling of the connectors also cools the end walls of the bushing, when a homogeneous temperature is required at the level of the bushing tip plate. Second, they create substantial thermal strain in the terminal ears which are fixed at both opposite ends of the bushing body because of varying levels of thermal expansion as the temperature varies over 1000° C. in a few decimeters, leading to warping of the ears and generation of substantial internal stresses. Furthermore the terminal ears are also exposed to vibrations generated by the electrical generator feeding electrical current for the heating of the ears or the flow of molten glass through the tip plate and transmitted to the terminal ears through the side walls. The combination of thermal strain and vibrations eventually leads to the failure by fatigue at high temperature of the terminal ears.
Reinforced ears have been proposed in the art, in order to prolong the service life of the ears. For example, U.S. Pat. No. 4,026,689 proposes a T-shaped terminal ear comprising a top plate, extending substantially horizontally from an end wall of a bushing assembly and a bottom plate, substantially normal to both top plate and bushing end wall, without contacting the latter. An electrical connector can be coupled to the free end of the bottom plate. The bottom plate also acts as stiffening rib to the top plate, but the welding between the top and bottom plates is quite sensitive to the thermal and fatigue strains to which the ear is exposed.
U.S. Pat. No. 4,634,460 discloses a terminal ear for heating a drain bushing for use in the start-up of a glass melting furnace. The terminal ear is reinforced with a gusset, which is a generally U-shaped strip or band having a central portion and two tapered legs extending at substantially right angles therefrom. The gusset is coupled to the terminal ear, providing support for the ear. Unlike bushings for attenuating glass fibres, comprising a tip plate with a multitude of small tips, a drain bushing comprises only a limited number of large drain holes. For these reasons, the ears and the gussets can be integrally connected to the bottom wall of the bushing as by welding, which is not the case with bushing assemblies for the production of glass fibres.
U.S. Pat. No. 3,512,948 discloses a bushing assembly comprising a terminal in the form of an inverted Y formed from two plate members of a platinum-rhodium alloy, the stem of which extending up the centre line of the end wall and being connected through the wall to an internal V-shaped grid. The arms of the Y extend across the end wall but terminate short of the edges. The line and angle of the arms are calculated to give a desired current distribution across that wall, and have the molten glass within the feeder, to eliminate “cold spots” in the corners of the feeder.
WO01/74727 discloses a terminal ear for conducting electrical current to a bushing, the terminal ear comprising: a conducting portion having a longitudinal axis, being coupleable to the bushing at a first end of said conducting portion, and having a first cross-section in a plane perpendicular to said longitudinal axis; and an elongate support portion, coupled to said conducting portion, extending substantially parallel to said longitudinal axis, and having a second cross-section in a plane perpendicular to said longitudinal axis, said second cross-section having a moment of inertia greater than a moment of inertia of said first cross-section. Although the terminal ear disclosed in WO01/74727 has a higher resistance to bending, its resistance to torsion and warping is not satisfactory.
There remains a need for an optimized design of a bushing terminal ear with enhanced resistance to thermal and fatigue strain and thus with prolonged service time thereof. This and other problems are solved by the present invention.